The present invention relates to the electrolytic production of aluminum. More particularly, the invention relates to the production of commercial purity aluminum with an electrolytic reduction cell including ceramic inert anodes.
The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable and dimensionally stable anodes. Replacement of traditional carbon anodes with inert anodes should allow a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also possible because inert anodes produce no CO2 or CF4 emissions. Some examples of inert anode compositions are provided in U.S. Pat. Nos. 4,374,050, 4,374,761, 4,399,008, 4,455,211, 4,582,585, 4,584,172, 4,620,905, 5,794,112, 5,865,980 and 6,126,799, assigned to the assignee of the present application. These patents are incorporated herein by reference.
A significant challenge to the commercialization of inert anode technology is the anode material. Researchers have been searching for suitable inert anode materials since the early years of the Hall-Heroult process. The anode material must satisfy a number of very difficult conditions. For example, the material must not react with or dissolve to any significant extent in the cryolite electrolyte. It must not react with oxygen or corrode in an oxygen-containing atmosphere. It should be thermally stable at temperatures of about 1,000xc2x0 C. It must be relatively inexpensive and should have good mechanical strength. It must have high electrical conductivity at the smelting cell operating temperatures, e.g., about 900-1,000xc2x0 C., so that the voltage drop at the anode is low and stable during anode service life.
In addition to the above-noted criteria, aluminum produced with the inert anodes should not be contaminated with constituents of the anode material to any appreciable extent. Although the use of inert anodes in aluminum electrolytic reduction cells has been proposed in the past, the use of such inert anodes has not been put into commercial practice. One reason for this lack of implementation has been the long-standing inability to produce aluminum of commercial grade purity with inert anodes. For example, impurity levels of Fe, Cu and/or Ni have been found to be unacceptably high in aluminum produced with known inert anode materials.
The present invention has been developed in view of the foregoing, and to address other deficiencies of the prior art.
An aspect of the present invention is to provide a process for producing high purity aluminum using inert anodes. The method includes the steps of passing current between a ceramic inert anode and a cathode through a bath comprising an electrolyte and aluminum oxide, and recovering aluminum comprising a maximum of 0.2 weight percent Fe, 0.1 weight percent Cu, and 0.034 weight percent Ni.
Another aspect of the present invention is to provide a method of making a ceramic inert anode that is useful for producing commercial purity aluminum. The method includes the step of mixing metal oxide powders, and sintering the metal oxide powder mixture in a substantially inert atmosphere. A preferred atmosphere comprises argon and from 5 to 5,000 ppm oxygen.
Additional aspects and advantages of the invention will occur to persons skilled in the art from the following detailed description thereof.